258 the teaching oe mechanics by expeeiment. 
supported on a central knife-edge and carrying at its extremities two 
pots or buckets which are themselves connected to it by knife-edges. 
Its stability can be adjusted by sliding a weight up and down a 
vertical rod which is attached to the beam. Its deflection can be 
read by means of a pointer upon a scale below and its period can of 
course be observed. The student measures the moment of inertia of 
the oscillating system in the first instance. There are two methods by 
which he may do it. One is to take the period as it swings and then 
after that is done to place known equal masses in each of the two 
pots at the ends of the beam and then again take the period, and by 
comparing the two periods and knowing the amount of mass that has 
been added he is able to determine the moment of inertia. This is 
because the knife-edges on which the beam itself is suspended and 
those on which the pots are suspend¬ 
ed are all in one plane, so that the 
addition of the masses in the pots does 
not affect the stability of the apparatus 
but only affects its moment of inertia. 
Another method by which he may do 
it is to take the period and then put a 
small weight in one pot only and 
observe the statical deflection to which 
it gives rise : from those two observa¬ 
tions he has the data needed to work 
out the moment of inertia. Having 
found the moment of inertia he can 
use the apparatus ballistically by drop¬ 
ping a mass from a height into one of 
the two pots; but a mass dropped in¬ 
to one pot only would produce a 
double effect, namely a dynamical 
effect and a statical effect, and in order 
that the statical effect should be pre¬ 
vented it is necessary that the other 
pot should beforehand be provided 
with a mass of the same weight. So 
I put in the first instance into the 
other pot a mass equal to the mass 
which is presently to be dropped into 
the other pot. It tends of course to 
deflect the beam. That deflection is 
prevented by its being brought up 
against a stop, but the stop is so ar¬ 
ranged that when the beam begins to 
swing the stop falls away and the 
beam is left perfectly free. A weight 
is now suspended from the stand at a 
definite height and allowed to drop in. Fi 'J- s - Tri f l ^ r Suspension 
The stop which held the beam from 
